D

Step 1: Define the term isochoric. The term 'isochoric' refers to a process that occurs at constant volume. This means that the volume of the system does not change during this thermodynamic process.

Step 2: Understand other processes:
- Isobaric: This process occurs at constant pressure. The volume can change while the pressure remains the same.
- Isothermal: This process occurs at constant temperature. The volume can also change as long as the temperature stays constant.
- Adiabatic: In this process, no heat is exchanged with the surroundings, and the volume can change, affecting the temperature.

Conclusion: Among the given options, only the isochoric process maintains a constant volume throughout.
Therefore, the correct answer is D.